In higher organisms, the highly specialized adipocyte is the primary repository of energy, stored as triacylglycerol in the intracellular lipid droplet. Although the enzymes involved in both the formation and hydrolysis (i.e., lipolysis) of lipid are known, the molecular processes by which metabolites traffic in and out of the droplet remain a mystery. The elucidation of these processes is important for understanding not only how organisms regulate their daily energy needs, but also for understanding disorders of excessive abnormal lipid storage such as obesity or the severe depletion of energy reserves, such as occurs with the cachexia of chronic illness. Moreover, abnormal lipid depositions have been identified in a number of pathophysiological conditions in which cells may assume adipocyte characteristics, including foam cells of atherosclerotic plaques (Fredrickson, et al., The Metabolic Basis of Inherited Disease (Stanbury, et al. (eds.), 4th Ed., pp. 604-655, McGraw Hill Book Co., Inc., New York (1978)); Minor, et al., J. Lipid Res. 30:189-197 (1989)) and, in a number of rare conditions, such as ichthyosis (Chanarin, et al., Brit. Med. J. 1:553-555 (1975); Williams, et al., J. Inher. Metab. Dis. 11:131-143 (1988)), in which a variety of cells become lipid-laden. One approach to understanding adipocyte function has centered on examining the fibroblastic cell lines that may be stimulated to differentiate into adipocytes in culture (Green, et al., Cell 1:113-116 (1974); Green, et al., Cell 5:19-27 (1975)), with special emphasis on describing proteins unique to adipocytes (see, e.g., Spiegleman, Trends Genet. 4:203-207 (1988) and Ringold, et al., Recent Prog. Horm. Res. 44, 115-140 (1988) for a review). Interest in such proteins is heightened by the evidence that fat cell size, which is directly related to lipid content, is determined by a "set point" possibly regulated by signals emanating from the adipose cell (Faust, et al., Science 197:393-396 (1977)).
The intracellular lipid droplet of adipocytes, the locus of stored food energy, has at its periphery a complex structure which alters with the developmental and physiological state of the cell. Electron microscopic studies have shown that the lipid droplets from both native and cultured cells are surrounded by a well developed network of filaments (Wood, Anat. Rec. 157:437-448 (1967); Novikoff, et al., J. Cell Biol. 87:180-196 (1980); and Franke, et al., Cell 49:131-141 (1987)) as well as endoplasmic reticulum cisternae and tubules (Slavin, Anat. Rec. 195:63072 (1979); Cushman, S. W., J. Cell Biol. 46:326-341 (1970); Blanchette-Mackie, et al., Int. J. Obesity 8:67-73 (1984)), which may extend into the core of the lipid droplet forming aqueous channels (Blanchette-Mackie, et al., Int. J. Obesity 8:67-73 (1984)). In metabolically active adipocytes, the luminal leaflet of the channels contains fatty acid products of triacylglycerol hydrolysis which are visualized under appropriate preparatory procedures as lamellar whorls (Blanchette-Mackie, et al., Int. J. Obesity, 8:67-73 (1984)) or lipid domains within the membrane leaflet (Amende, et al., Cell Tissue Res. 247:85-89 (1986)). Franke, et al. found by immunofluorescence that the intermediate filament protein, i.e., vimentin, surrounds the lipid droplet, and they found by electron microscopy that regularly spaced intermediate filament-like fibrils are located at the lipid periphery (See, Cell 49:131-141 (1987)). It was concluded that the lipid droplet is encaged in a vimentin-containing structure.
For several years, hormonal control of metabolic processes and protein phosphorylation in isolated adipocytes have been investigated. Recently, a prominent phosphoprotein in whole cell extracts was identified that is a substrate for cAMP-protein kinase (i.e., A-kinase) (Egan, et al., J. Biol. Chem. 265:18769-18775 (1990)). Using this phosphoprotein as a model A-kinase substrate, evidence was presented that in the intact cell, insulin stimulates the dephosphorylation of the protein by a mechanism independent of insulin's ability to lower cAMP, i.e., insulin activates a phosphatase that removes those phosphates inserted by A-kinase.
As of yet, however, the metabolic processes by which metabolites traffic in and out of the lipid droplet still remain a mystery. Moreover, very little is known about the biochemical composition of the surface of such lipid droplets. In order to truly understand the biochemical make-up of the surface of the lipid droplet and the role it plays in allowing metabolites to traffic in and out of the lipid droplet, there exits a need for specific knowledge of the proteins or other molecules on the surface that may be involved in lipid metabolism and trafficking. As previously mentioned, abnormal lipid depositions have been identified in a number of pathophysiological conditions in which cells assume adipocyte characteristics, (e.g., foam cells of atherosclerotic plaques) and, in a number of rare conditions (e.g., ichthyosis) in which a variety of cells become lipid-laden. As such, there exits a need for a definitive marker which can differentiate true adipocytes from non-adipocyte cells which, as a result of pathophysiological conditions, assume adipocyte characteristics and become lipid-laden. The present invention remedies these needs.